By way of example, a driver laser arrangement for an EUV light source was disclosed in US 2009/0095925 A1. The driver laser arrangement described therein comprises a beam source for generating pulsed laser radiation and one or more optical amplifiers for amplifying the pulsed laser radiation. The beam source of the driver laser arrangement serves to produce so-called seed pulses, which are amplified in the optical amplifier or amplifiers to high laser powers of several kW, possibly of 10 kW or more. The laser radiation amplified by the driver laser arrangement is fed via a beam guiding device to a focusing device, which focuses the laser radiation or the laser beam in a target region. A target material which transitions into a plasma phase when irradiated by the laser beam and emits EUV radiation in the process is provided in the target region.
In the driver laser arrangement described above, a pre-pulse and, following in succession after a short period of time, a main pulse are typically produced by the beam source and focused on the target region with the target material. The pre-pulse is intended to serve to influence the target material, for example to heat, expand, vaporize or ionize the latter and/or to produce a weak, or possibly a strong, plasma. The main pulse is intended to serve to convert the majority of the material influenced by the pre-pulse into the plasma phase and generate EUV radiation in the process. The pre-pulse typically has significantly lower laser power than the main pulse. The same laser wavelength is used for the pre-pulse and the main pulse in the driver laser arrangement from US 2009/0095925 A1. However, it is also possible to use different wavelengths for the pre-pulse and the main pulse, as described in WO 2011/162903 A1, in which a seed laser is used to produce the pre-pulse and a further seed laser with a different wavelength is used to produce the main pulse, which pulses are combined by means of a beam combiner in order to pass along a common beam path through one or more amplifiers and the beam guiding device following the driver laser arrangement.
In the driver laser arrangement described above, there can be a reflection of the amplified laser radiation, for example, at the target material, which may, e.g., be present in the form of tin droplets. The back-reflection produced at such a droplet returns into the optical amplifier or amplifiers and passes through the gain medium present there, and so the back-reflection is also amplified in the optical amplifier or amplifiers. Even a weak back-reflection may be sufficient to generate power after amplification in the gain medium of the optical amplifier that can damage the optical or possibly mechanical components in the optical amplifier or in the beam path upstream of the optical amplifier.
In order to suppress back-reflected laser radiation, it is known to use so-called optical isolators, which only let laser radiation pass in one direction and which are also referred to as optical diodes on account of this property. Such optical isolators can be arranged between a beam source and an optical amplifier or else between two optical amplifiers. By way of example, DE 41 27 407 A1 discloses the practice of respectively applying one optical diode between an injection seeding laser and a resonator and between a resonator and an amplifier.
In the driver laser arrangement described above, high laser powers of 500 W and more, of 1 kW and more and even of 10 kW and more can be produced during amplification. A problem in the case of such high laser powers is that the optical components used in conventional optical isolators may cause strong thermally-induced aberrations, in particular astigmatism, and moreover may possibly be damaged by the laser radiation. Moreover, there is a problem in that optical isolators generally are unable to completely suppress laser radiation propagating in the undesired direction, and so the non-suppressed power component in the case of high laser powers is so large despite the use of optical isolators that returning laser radiation is produced, possibly even though an optical isolator is used. Compounding this in the current application is that optical diodes based on the principle of the polarizer or phase shifter can only suppress laser radiation with a specific phase jump or with a specific phase shift (e.g., 180°) for design reasons. The value for this phase jump or for this phase shift may not be met in the case of reflection at a droplet, and so the laser radiation reflected at the droplet cannot be completely suppressed by the optical diode for this reason either.